The wire grid polarizer (WGP) based Liquid Crystal on Silicon (LCoS) microdisplay projection system (MDPS) [C. Pentico, M. Newell and M. Greenberg, “Ultra high contrast color management system for projection displays,” SID 03 Digest, pp. 130-133, 2003; also U.S. patents, Kurtz et al, U.S. Pat. No. 6,585,378 and Pentico et al. U.S. Pat. No. 6,857,747] realizes both high resolution and high image contrast in comparison to other micro-display projection display technologies (such as transmissive Liquid Crystal Micro-Display (xLCD) and Digital Light Processor (DLP)) and direct-view display panels. The lack of on-screen brightness is mitigated by the use of three microdisplay panels, one for each primary color band. An example of the WGP-based projection system is given in FIG. 1. The light from a high-pressure discharge lamp is homogenized by a long light rod (pipe). The light may also be preferentially polarized or have its unused portion recycled. A spatially uniform light distribution at the exit of the light pipe is imaged by a series of lenses, fold mirrors and dichroic band splitters to one or more LCoS panels (in a one-, two-, three- and four-panel polarization based MDPS). In a WGP-based MDPS, the primary function of the wire-grid polarizers is to separate the outgoing light beam from the incoming light beam [D. Hansen, E. Gardner, R. Perkins, M. Lines, and A. Robbins, “The display applications and physics of the Proflux wire grid polarizer,” SID 02 Digest, p. 730, 2002]. In this respect, the WGP is typically tilted at ±45° with respect to the principal direction of light propagation in a given LCoS panel illumination arm. The return light from each LCoS panel is then steered (deflected) to an orthogonal path, relative to the incoming illumination. The WGP also serves as a polarizing device. The WGP is a grid-polarizer, namely, it transmits a linear polarization aligned orthogonal to the direction of parallel micro-wires and reflects a complementary linear polarization parallel to the direction of the wires. When used in off-normal incidence, the WGP is configured as a high polarization-contrast mode if the transmitted linear polarization is contained in the plane of incidence (P-plane). In the diagram of FIG. 1, this high contrast configuration requires the micro-wires to be oriented parallel to the S-plane (orthogonal to the plane of incidence with respect to the central ray). The wires are aligned perpendicular to the plane of drawing in FIG. 1. Due to the trade-off between brightness and aperture of optical system (“Etendue”), the MDPS also requires the use of a moderate numerical aperture of each optical element. It is typical to configure the optical elements to function well with f/2.4 system (approximately ±12° in air incidence). The P- and S-plane of polarizations then refer to the linear polarization of the central ray in the cone bundle (hereafter term the principal ray) with respect to each local WGP element.
In the 3-panel WGP-based MDPS shown in FIG. 1, the images displayed by LCoS panels 15, 15a and 15b (corresponding to red, green and blue color channels, respectively) are aggregated (converged) by an X-cube 19 and then projected to a large screen. Each color channel has an LCoS panel, a WGP, either tilted at +45°, or at −45°, as a beam splitter and polarizer/analyzer as well as a dedicated trim retarder compensator: 21, 21′ and 21″, each associated to panel 15, 15a and 15b, respectively. Each color channel also has its own pre-polarizer oriented to transmit P-polarization (one or more elements of WGPs or dichroic sheet polarizers oriented at normal incidence with respect to the principal ray; these are not shown in FIG. 1) and clean-up analyzer oriented to transmit S-polarization (one or more elements of WGPs or dichroic sheet polarizers oriented at normal incidence with respect to the principal ray; these are also not shown in FIG. 1).
The trim retarder compensator is the crucial optical element in each color channel of the MDPS. It removes the residual LCoS panel retardance in the panel off-state [D. Anderson and K. Shahzad, “Off-axis LCoS compensation for enhanced contrast,” SID 03 Digest, pp. 1433-1435, 2003]. The residual LCoS panel retardance can be categorized into in-plane (also A-plate) and out-of-plane (also C-plate) components. Here, the term “retardance’ refers to linear retardance magnitude unless stated otherwise. Linear retardance causes a phase difference to two orthogonal linear polarizations, aligned parallel to the extra-ordinary and ordinary axes of the linear retarder. There is also a type of retardance called “circular retardance” which causes a relative phase difference of right- and left-handed circular polarized light. Normal incidence rays in the cone bundle see only the A-plate retardance, whereas off-axis rays (both oblique, i.e. non-normal but along the principal S- and P-planes; and skew, i.e. non-normal and incident away from the principal S- and P-planes) experience the C-plate retardance in addition to the A-plate retardance. A-plate retardance is not seen at the trivial situation of 90° ray angle in the birefringent medium).
In the nominal A-plate compensation scenario, the A-plate retardance of the compensator matches that of the LCoS panel in the off-state. The slow axes of both compensator and the LCoS panel are configured at orthogonal azimuthal offset (termed “crossed axes”). The same applies to the two fast axes. The role of the fast/slow axes switches from the trim retarder compensator element to the LCoS panel element for normal incidence light. The light of a given linear polarization is alternately delayed more then less, or vice-versa in the two successive elements. The net effect is zero relative delay for the incoming polarization. Thus the output polarization from the pair of trim retarder and panel in the off-state is unchanged versus their input polarization. This output light is then rejected by the combination of WGP and clean-up polarizer, whereby the high-reflection axis for the WGP and the high-transmission axis for the clean-up polarizer are at orthogonal orientation to the incoming polarization to the trim retarder and panel pair. The illumination for dark-state panel then does not appear on the screen. The introduction of the trim retarder as a compensator also does not alter significantly the throughput of the panel on-state. Hence the sequential contrast (full on/full off) is excellent.
In practice, the A-plate retardance of both the LCoS and the compensator show a range of values due to manufacturing tolerances in device thickness and material birefringence control as well as operational drifts (temperature, mechanical stress etc). It is then typical to provide for a higher A-plate retardance in the compensator than the value of the nominal LCoS panel retardance [J. Chen, M. G. Robinson and G. D. Sharp, “General methodology for LCoS panel compensation,” SID 04, Digest, pp. 990-993, 2004]. For example, a vertical aligned nematic (VAN) LCoS may exhibit a 2 nm A-plate retardance (at λ=550 nnm) whereas the trim retarder compensator might be fabricated with a 5 nm (at λ=550 nnm) A-plate retardance. This mismatch in A-plate value requires offsetting of the optic axis of the compensator, relative to the nominal crossed axes configuration of trim retarder compensator/LCoS panel pair. With a VAN-LCoS, the slow axis of the panel is typically configured substantially parallel to the bisector of the S- and P-planes (i.e., slow axis at ±45° and ±135° where the P-polarization is parallel to 0°/180° and S-polarization is parallel to ±90°). This configuration is crucial to utilizing the VAN-LCoS panel as an efficient electrically-controlled birefringence (ECB) device, with the crossed polarization conversion for this reflective device is given by:I(output crossed polarization)=I(input linear polarization)*[sin(Δnd/λ)*sin(2φ)]2,where Δnd is the single-pass retardance of the VAN-LCoS panel; λ is the illumination wavelength and φ is the orientation of the slow-axis relative to the P-polarization. As a consequence of the ECB requirement, VAN-LCoS is typically configured as an approximate quarter-waveplate retarder in single pass (in the panel on-state) and its slow/fast axes approximately bisect the S- and P-polarization planes.
For the purpose of describing the invention, references will be made to a single color-channel in VAN-mode LCoS microdisplay projection core optics. The single channel description is part of a one or more panel WGP-based microdisplay projection systems. Also, the pre-polarizer before the WGP and the clean-up polarizer after the WGP reflection is aligned non-tilted, with respect to the principal ray propagation direction. The pre-polarizer comprises one or more stages of substantially parallel elements of grid-based (reflective) polarizers (such as aluminum wire grid) or regular dichroic sheet (absorptive) polarizers. The clean-up polarizer comprises one or more stages of substantially parallel absorptive polarizer elements.
A schematic representation of the core optics 200 of either the red or the blue channel in FIG. 1 light engine is depicted in FIG. 2. The cone of light output from the prior stage light pipe (or other homogenizer such as Fly's Eye Array), either unpolarized or partially polarized, is linear polarized by the pre-polarizer 201. The transmission axis of this polarizer 220 is substantially parallel to the transmission axis of the subsequent WGP element 202. This linear polarization direction is termed “P-polarization”, with reference to the principal ray and the conical mount of the WGP element. The WGP element is said to have been rotated at +45° about the +Y-axis and with respect to the +Z-axis (or simply tilted at +45° w.r.t. Z-axis). This adheres to the convention of Euler angle rotation with a Right-Handed XYZ coordinate system (RH-XYZ). Similarly, the core optics of the green channel (not shown) has the WGP tilted at −45° w.r.t. Z-axis and the return pass to the WGP being steered towards a clean-up polarizer positioned at the reflected port of WGP in return pass.
The micro-wires on the surface of the WGP element 202 are aligned parallel to the Y-axis in the drawing. The wires are located on the rear-side of the WGP substrate (away from the input) such that the linear polarized light is less affected by the thermal and mechanical stress-induced birefringence in the substrate. After a complete double pass, having traversed the parallel stages of the trim retarder compensator (TR) 203 and the VAN-LCoS panel 204, the beam is analyzed by the WGP element. The orthogonal polarization, S-polarization, as reflected by the wire-side of the WGP element is deflected towards the clean-up polarizer 205, having a transmission axis orthogonal to the pre-polarizer. The analyzer polarization is shown as 221. This reflected light does not pass through the WGP substrate and hence is less affected by the induced birefringence in the substrate.
The LCoS panel has been shown with its slow-axis (SA) 230 located in the first quadrant of a RH-XYZ coordinate system, while looking at the beam coming to the observer in the first pass (RH-XYZ). In describing the SA of a VAN-LCoS panel, reference is made to the azimuthal orientation of the SA with a polar angle tilt towards +z axis (positive tilt). In this prior art example shown, the LCoS SA is given by the azimuthal angle 235, counter-clockwise (CCW) from the X-axis being positive angles. The fast-axis (FA) of the VAN-LCoS panel is defined as being orthogonal to the SA orientation (i.e., ±90° azimuthal offset to SA). This FA 231 is shown as being located in quadrant 2 and 4, at +135°/−45° azimuthal angles from the X-axis. The trim retarder compensator 203 in the case of higher-value retardance has to be rotated or clocked to orient its SA in the quadrant neighboring the LCoS SA, so that the two sets of slow axes are not crossed. An example of a generic trim retarder compensator is shown as element 203 with its slow-axis 240 oriented at the azimuthal angle 245. For a moderately higher trim retarder compensator retardance and a rather low VAN-LCoS panel retardance, the trim retarder compensator SA can typically deviate up to 30° from the closest S- or P-axis, although a deviation of less than 15° is preferable. The terms SA and FA when used for both VAN-LCoS panels and trim retarder compensators here refer to the two orthogonal birefringent axes when the linear retardance is measured at normal incidence. The SA and FA orientations change with off-axis illuminations, as well as reversing of SA/FA roles for a negative out-of-plane retardance component at a sufficiently large angle of incidence.
In the prior-art disclosures, the optimal trim retarder compensator incorporates an A-plate element and a −C-plate element (out-of-plane retardance with negative sign of birefringence). This trim retarder compensator is aligned substantially parallel to the LCoS X-Y plane. The requirements for a good trim retarder compensator device are well known [see for example K. Tan et al., “Design and characterization of a compensator for high contrast LCoS projection systems,” SID 2005, p. 1810, 2005]. There are a variety of materials being used to realize the compensator A-plate and −C-plate retardance. Traditionally, an isotropic polymer is stretched either in one or two axes and the resultant biaxial or uniaxial negative layer can be used to fully compensate for LCoS panel retardance [H. Mori, et al., “Novel optical compensation method based upon a discotic optical compensation film for wide-viewing-angle LCDs,” SID 03 Digest, p. 1058, 2003].
More recently, liquid crystal mixture crossed linked into a polymer host (LCP) has been shown to be more versatile in terms of reliability, uniformity and ease of targeting retardance values [Zieba et al. US Patent Application Publ. No. 20050128380]. The LCP layer is integrated with inorganic thin-films to realize the −C-plate components [Tan et al, US Patent Application Publ. No. 20050128391]. The full function trim retarder compensator has been shown to provide compensation for excellent contrast as well as being environmentally stable [M. Duelli et al., “High performance contrast enhancing films for VAN-mode LCoS panels,” SID 05 Digest, p. 892, 2005].
The invention disclosed here employs a form birefringent film tilted at an angle to compensate the retardance of a reflective LCoS or transmissive LC device in the dark-state resulting in significant improvement in contrast. The birefringent film has a uniaxial indicatrix and it is configured with its C-axis parallel to the device normal.
Such a birefringent component does not contain organic materials and consequently avoids reliability failures or contrast degradation over time which are inherent risks with organic birefringent devices in prior-art retarder applications. A C-plate-only retarder is a birefringent element where the axis of optical symmetry lies along the device normal of a substantially parallel plate. A C-plate retarder does not present any net retardation for normal-incidence rays. For off-normal rays, extra-ordinary rays (e-wave), the effective index of refraction can be higher or lower value than the index value of the orthogonal, ordinary ray (o-wave) polarization. This means the C-plate can possess either a positive C or a negative C retardance.
In addition to the reliability improvement over the prior-art retarder technologies, the use of C-plate-only retarder at a tilted alignment is advantageous for substantially reducing the retarder cost by reducing the number of elements in the optical system as well as simplifying assembly.